We have isolated two subtypes of helper T cells from human tonsils: CD4⁺ CD57⁺ cells, mostly located in the germinal center (GC), and CD4⁺ CD57^- cells, distributed through the interfollicular areas but also present in the GC. In a functional study, we have compared the capacities of these T-cell subtypes to stimulate B cells in cocultures. In order to block T-cell proliferation while maintaining their activation level, we pretreated isolated T cells with mitomycin C prior to culture in the presence of B cells and added polyclonal activators such as PHA and Con A, combined or not with IL-2. Contrary to CD4⁺ CD57^- cells, CD4⁺ CD57⁺ cells did not markedly enhance B-cell proliferation. Even when sIgD^-B cells typical of germinal center cells were tested, the CD4 CD57 cells had no significant effect. This is in accordance with the location of these cells: They mainly occupy the light zones of the GC where few B cells divide. Even when added to preactivated, actively proliferating cells, CD4⁺ CD57⁺ cells failed to modulate B-cell multiplication. On the supernatants of B-cell-T-cell cocultures, we examined by the ELISA technique the effect of T cells on Ig synthesis. Contrary to CD57^- T cells, whose effect was strong, CD57⁺ T cells weakly stimulated Ig synthesis. More IgM than IgG was generally found. Because CD57 antigen is a typical marker of natural killer cells, we tested the cytolytic activity of tonsillar CD4⁺ CD57⁺ cells on K562 target cells. Unlike NK cells, neither CD4⁺CD57⁺ nor CD4⁺ CD57^- cells exhibit any cytotoxicity. Thus, germinal center CD4⁺ CD57⁺ cells are not cytolytic and do not strongly stimulate either B-cell proliferation or Ig secretion. CD4⁺ CD57^- cells, however, enhance B-cell proliferation and differentiation, thus acting like the classical helper cells of the T-dependent areas.